The present invention relates to a fiber drawing method of optical fiber capable of suppressing diameter fluctuations and to optical fiber drawing furnaces used of in this method.
Optical fibers are normally fabricated by softening with heat and drawing from an optical fiber preform shaped like a rod in the optical fiber drawing furnace. In order to reduce the production cost of optical fibers, it is effective to increase the length of the preform and thereby decrease the number of replacement works thereof. At the present time it is possible to make an optical fiber of the total length several hundred kilometers by single optical fiber drawing.
The drawing furnaces have also been improved in order to implement stable drawing of such an elongated fiber preform. The drawing furnace disclosed in Japanese Patent Application Laid-Open No. H09-2832 (which will be called hereinafter a prior art) is an example of such drawing furnaces for drawing of the elongated fiber preform. This drawing furnace is constructed in such structure that a preform container cylinder is coupled to an upper portion of a furnace core tube provided with a heater surrounding it. Then the fiber preform is set in the preform container and the lower end thereof is guided into the furnace core tube. On the other hand, an inert gas such as helium or nitrogen is supplied from the upper end of the preform container. This keeps the furnace core tube and a semi-closed space above it (which will be referred to hereinafter simply as a semi-closed space) in a non-oxidizing atmosphere and the fiber preform is heated to soften from the lower end by the heater, followed by drawing.
During the drawing operation of optical fiber, the fiber preform becomes shorter and shorter with progress of fiber drawing. In the case of the drawing furnace with the preform container coupled, which is disclosed in this prior art, as the fiber preform becomes smaller with progress of fiber drawing, the space gradually becomes wider between the preform container and the fiber preform. It makes the inert gas in this space easier to flow and also increases a temperature difference between the inert gas in this space and the inert gas present between the furnace core tube and the fiber preform under drawing, so as to cause convection of the inert gas in the semi-closed space.
Occurrence of such convection leads to instable flow of the atmosphere near the lower end of the fiber preform in the softened state with heat, i.e., near the outside of the opening end of the semi-closed space. It can affect the optical fiber under drawing so as to make the diameter fluctuations of optical fiber considerably large, thus making it difficult to obtain products with desired quality.
As countermeasures against it, the prior art discloses the technology of disposing an annular auxiliary heater around the upper end of the preform container and heating and retaining the inside of the upper end of the preform container at several hundred degrees. It is described that this technology can prevent occurrence of the convection in the semi-closed space and thus permit the optical fiber to be drawn in steady diameter.
With the drawing furnace as above described, further increase in the length of the fiber preform will result in also extending the preform container housing it. It also increases the volume of the semi-closed space. It is obvious that the heating region by the auxiliary heater also has to be elongated in order to prevent the unwanted convection in the semi-closed space.
The fiber preform of this kind is supported so as to be suspended from a support rod having the diameter smaller than the outside diameter of the preform in the preform container. The preform has a shoulder gradually decreasing its diameter toward the end, near a joint with the support rod. When the fiber preform is heated for drawing, this shoulder radiates a large quantity of heat, which also heats up the preform container facing it. With the elongation of the fiber preform accompanied by the expansion of the heating region inside the preform container, there is a possibility of overheating the internal wall of the preform container and eventually melting it. The shoulder of the fiber preform can also soften by the overheat, whereby the fiber preform undergoes axial extension in the shoulder part because of the weight of the fiber preform itself, so as to raise a possibility of failure in normal drawing of optical fiber.
In view of the above problem, an object of the present invention is to provide an optical fiber drawing method and an optical fiber drawing furnace capable of surely producing the optical fiber in steady diameter even in cases using the elongated fiber preform.
In order to accomplish the above object, an optical fiber drawing method according to the present invention is a drawing method of optical fiber comprising steps of setting an optical fiber preform in a furnace core tube and a preform container connected to an upper portion of the furnace core tube and drawing an optical fiber from one end of the preform by softening with heat, wherein an upper portion of the preform container is provided with an auxiliary heater and cooling means for cooling the upper portion of the preform container, and the drawing step includes adjusting a cooling quantity by said cooling means.
Namely, a drawing furnace used in this drawing method is a fiber drawing furnace comprising a furnace core tube through which an optical fiber preform penetrates vertically, a heater disposed around this furnace core tube and a preform container connected to an upper portion of the furnace core tube so as to be integral with the furnace core tube to form a semi-closed space opening in part at a lower end, for housing the fiber preform inside, the fiber drawing furnace further comprises an auxiliary heater disposed at an upper portion of the preform container and cooling means for cooling the upper portion of the preform container.
The present invention permits the temperature difference to be reduced in the space of clearance to the fiber preform in the semi-closed space formed by the furnace core tube and the preform container, so as to suppress occurrence of convection described above, even in the case of the elongated fiber preform. Further, cooling the upper portion of the preform container prevents the overheat of the internal wall of the preform container and, in turn, prevents the overheat of the shoulder of the fiber preform, which permits the optical fiber to be surely drawn in steady diameter and which prevents breakage of the drawing furnace.
Here the drawing furnace is preferably one further comprising at least one temperature sensor for measuring an internal temperature of the upper portion of the preform container and adjusting the cooling quantity based on the temperature measured by the temperature sensor.
It is preferable to employ either of the following techniques for the cooling quantity from the upper portion of the preform container.
For example, the cooling quantity may be adjusted by supplying cooling air into clearance between the auxiliary heater and the outer wall of the preform container. Another technique is to adjust the cooling quantity by heater moving means for moving the auxiliary heater to change the distance to the preform container. In this case, it is also optional to supply the cooling air into the clearance between the preform container and the auxiliary heater, which is created by movement of the auxiliary heater.
In another technique, the auxiliary heater has a heating element and a heat insulator formed around it and difference to be reduced in the space of clearance to the fiber preform in the semi-Closed space formed by the furnace core tube and the preform container, so as to suppress occurrence of convection described above, even in the case of the elongated fiber preform. Further, the adjustment of the quantity of heat dissipation from the upper portion of the preform container prevents the overheat of the internal wall of the preform container and, in turn, prevents the overheat of the shoulder of the fiber preform, which permits the optical fiber to be surely drawn in steady diameter and which prevents breakage of the drawing furnace.
Here the drawing furnace is preferably one further comprising at least one temperature sensor for measuring an internal temperature of the upper portion of the preform container and adjusting the quantity of heat dissipation, based on the temperature measured by the temperature sensor.
It is preferable to employ either of the following techniques for the adjustment of the quantity of heat dissipation from the upper portion of the preform container.
For example, the quantity of heat radiation may be adjusted by supplying cooling air into clearance between the auxiliary heater and the outer wall of the the cooling quantity is adjusted by moving the heat insulator to change the distance to the preform container. In this case, it is also optional to supply the cooling air into the clearance between the heat insulator and the auxiliary heater, which is created by movement of the heat insulator.
In another technique, the furnace may further comprise a cooling fluid circulation path which is formed around the preform container and in which a cooling fluid flows, and supply means for supplying the cooling fluid into the circulation path. This cooling fluid is preferably air or water.